Calorimetric studies of flavin-binding proteins: FMN and FAD binding to hen egg riboflavin-binding proteins

Evaluation of riboflavin binding protein domain interaction using differential scanning calorimetry

Riboflavin binding (or carrier) protein (RfBP) is a monomeric, two-domain protein, originally purified from hens' egg white. RfBP contains nine disulfide bridges; as a result, the protein forms a compact structure and undergoes reversible three-state thermal denaturation. This was demonstrated using a differential scanning calorimetry (DSC) method [Wasylewski M. (2000) J. Prot. Chem. 19(6), 523-528]. It has been shown that the RfBP complex with riboflavin denaturates in a three-state process which may be attributed to sequential unfolding of the RfBP domains. In case of apo RfBP, the ligand binding domain denaturates at a lower temperature than the C-terminal domain. Ligand binding greatly enhances the thermostability of the N-terminal domain, whereas the C-terminal domain thermostability is only slightly affected and, in case of the examined holo RfBPs, the denaturation peaks of both domains merge or cross over. The magnitude of the changes depends on ligand structure. A detailed study of protein concentration effects carried out in this work allowed to estimate not only the thermostability of both domains but also the strength of domain interactions. The DeltaCp, of denaturation was found for C-terminus and N-terminus of RfBP-riboflavin complex to amount to 2.5 and -1.9 kcal mol(-1), respectively. The calculated domain interaction free energy, DeltaGCN, was estimated to be approximately -1580 cal mol(-1) at 67.0 degrees C. This value indicates that the interdomain interaction is of medium strength.

Binding study of riboflavin-binding protein with riboflavin and its analogues by differential scanning calorimetry

Thermal unfolding parameters of hens' egg-white riboflavin-binding-protein (RBP) were measured by differential scanning calorimetry. Thermal denaturation scans of apoRBP and RBP complexes with riboflavin and its analogues (FMN, N10 DL-glyceryl isoalloxazine, and N10 omega-hydroxypentyl isoalloxazine) have been measured. It was found that ligand binding causes increase of RBP thermal stability, as manifested by a change of denaturation temperature from 60.8 degrees C for apoRBP to 72.8 degrees C for RBP-Rf complex. For RBP-FMN complex, the denaturation temperature of 73.0 degrees C was even higher than for the RBP-Rf complex. The other two flavin analogues showed transition temperatures in between 66.9 degrees C and 68.8 degrees C, respectively. Analysis of excess heat capacity data showed that the best fit was the sum of two independent thermal transitions. One of the transitions, which contributed approximately 70% to the total heat effect, has transition temperature in the broad range of 60.5-73.2 degrees C; the other transition temperature is in the narrower range of 65.4-71.1 degrees C. The observed transitions can be related to RBP domains.

Error analysis in titration microcalorimetry of biochemical systems

A simplified method for titrations of biochemical systems is described as well as extensive error propagation through the data analysis. This work uses a Tronac Model 450 isoperibol titration calorimeter. Sample volumes of 2 ml are used and total heats of less than 5 mcal can be routinely measured. The binding of 3'-CMP to bovine pancreatic ribonuclease A is used to illustrate the methods. The binding enthalpy can be determined with a standard deviation of 1.5% and the free energy with a standard deviation of 2 to 3%.

Calorimetric studies of flavin binding protein: flavin analog binding

The effect of flavin structure variation upon the overall binding reaction of flavin to hen egg white riboflavin binding protein (WRBP) was correlated to thermodynamic parameters obtained via titration calorimetry. This effect was measured by determining a reference binding enthalpy (delta Href) for 3-carboxymethylriboflavin and subsequently comparing delta Href to the binding enthalpies determined for all other flavin analogs in this study. The reference enthalpy change was measured in detail under a variety of temperatures (14, 25, and 38 degrees C), pH's (5.5, 6.5, 7.4, 8.5), and buffer types. The reference enthalpy for the binding of N(3)-carboxymethylriboflavin to WRBP was measured as delta H = -20.9 kcal/mol at 25.0 degrees C and pH 7.4. Evaluation of the binding thermodynamic values for the reference flavin indicates that there is no H+ flux concomitant with binding. A small negative change in heat capacity (delta Cp) occurs during the binding process suggesting a contraction of the protein. Finally, the reaction appears to be independent of pH and buffer type indicating little, if any, involvement of charged residues in the binding of flavin to apoWRBP. Thermodynamic values for the binding of an additional eight flavin analogs were then measured at pH 7.4 and 25.0 degrees C. The thermodynamic binding parameters for these analogs were evaluated by comparison to those determined for the reference flavin. The results indicate that the ribityl C2' position is a major influence in the interaction between the ribityl side chain and the protein; the C9 position of the isoalloxazine ring is sterically restricted in a manner similar to positions C7 and C8; and the positions C7 and C8 appear to be unequal with regard to enthalpy release, suggesting that the C8 position is the most restricted region in the aromatic ring. These various findings indicate the unique ability of titration calorimetry to evaluate structural variation of ligands to their corresponding binding site interactions.